Thermography logging tool

ABSTRACT

The invention relates to a logging tool for determining the properties of a fluid surrounding the tool arranged downhole in a casing comprising a wall and having a longitudinal extension. The logging tool has a substantially longitudinal cylindrical shape with a longitudinal axis, and the logging tool comprises a sensor unit comprising an anemometer having a resistance probe electrically connected with three other resistors, a voltmeter and an amplifier for forming a bridge circuit, such as a Wheatstone bridge, having bridge current and bridge voltage. The invention further relates to a method for determining the properties of a fluid by means of the logging tool.

TECHNICAL FIELD

The present invention relates to a logging tool for determining theproperties of a fluid surrounding the tool arranged downhole in a casingcomprising a wall and having a longitudinal extension. The logging toolhas a substantially longitudinal cylindrical shape with a longitudinalaxis, and the logging tool comprises a sensor unit comprising ananemometer having a resistance probe electrically connected with threeother resistors, a voltmeter and an amplifier for forming a bridgecircuit, such as a Wheatstone bridge, having bridge current and bridgevoltage. The invention further relates to a method for determining theproperties of a fluid by means of the logging tool.

BACKGROUND

During oil production, it may be useful to be able to determine thefluid properties of a fluid in order to optimise the production process.Samples may be taken during production, or a tool able to conductcertain measurements may be loaded into the well.

One of such tools is disclosed in U.S. Pat. No. 5,551,287. In this tool,a constant temperature anemometer is used to determine the velocity of afluid. However, in order to measure the velocity, the instrument must becalibrated to ensure that a change in the resistance and thus in thetemperature of the sensor depends only on the velocity.

When measuring the velocity several places in a well, e.g. in the sidetracks, the fluid property changes from one position in the well toanother, and the tool needs calibration from position to position in thewell, or the measurements are inaccurate.

DESCRIPTION OF THE INVENTION

It is an object of the present invention to wholly or partly overcomethe above disadvantages and drawbacks of the prior art and provide animproved logging tool providing more accurate measurements of the fluidproperties.

The above objects, together with numerous other objects, advantages, andfeatures, which will become evident from the below description, areaccomplished by a solution in accordance with the present invention by alogging tool for determining the properties of a fluid surrounding thetool arranged downhole in a casing comprising a wall and having alongitudinal extension, the logging tool having a substantiallylongitudinal cylindrical shape with a longitudinal axis, wherein thelogging tool comprises:

-   -   a sensor unit comprising an anemometer having a resistance probe        electrically connected with three other resistors, a voltmeter        and an amplifier for forming a bridge circuit, such as a        Wheatstone bridge, having bridge current and bridge voltage,        wherein the sensor unit comprises switches for disconnecting or        connecting the amplifier and connecting or disconnecting a        voltage supply.

The anemometer may be a hot wire anemometer or a hot film anemometer.

Furthermore, the probe may be arranged on an outer face of the tool.

Moreover, the sensor unit may comprise a plurality of anemometers allhaving a resistance probe.

Additionally, the probes may be arranged on the outer face of the tool.

In one embodiment, the tool may comprise at least one thermocouplearranged partly on the outer face of the tool.

In another embodiment, the tool may comprise a plurality of electrodesarranged spaced apart around the longitudinal axis in the periphery ofthe tool, enabling the fluid to flow between the electrodes and thecasing wall.

The logging tool may further comprise a positioning device fordetermining a position of the logging tool along the longitudinalextension of the casing.

Furthermore, the logging tool may have a space between every twoelectrodes, which space is substantially filled with a non-conductivemeans.

Furthermore, the logging tool may comprise a driving unit for moving thetool in the casing.

In one embodiment, the invention relates to the logging tool asdescribed above, wherein

-   -   a temperature of the probe resistance is maintained constant by        varying the current by means of the amplifier so that a first        bride voltage is substantially zero when a first measurement of        a second bridge voltage is performed by the voltmeter, and    -   the second bridge voltage is an alternating voltage when a        second measurement of the first bridge voltage is performed.

In another embodiment, the second bridge voltage may be maintainedconstant, at a value different from zero, by the voltage supply when athird measurement of the first bridge voltage is performed.

In yet another embodiment, a third measurement may performed by usingthe thermocouples.

The alternating voltage or alternating current may have sine, square,rectangular, triangle, ramp, spiked or sawtooth waveforms.

In an embodiment, the logging tool may further comprise an electricalmotor powered by a wireline.

The invention also relates to a method for determining the properties ofa fluid by means of the logging tool according to any of the precedingclaims, comprising the steps of:

-   -   submerging the logging tool into a casing,    -   maintaining a constant temperature in the resistance probe by        varying the current,    -   increasing the bridge current by means of the amplifier if the        temperature in the resistance probe decreases until a first        bridge voltage is substantially zero,    -   measuring a second bridge voltage by means of the voltmeter,    -   disconnecting the amplifier and the voltage supply and        connecting a second voltmeter by means of the switches,    -   providing the second bridge voltage as an alternating voltage,        and    -   measuring the first bridge voltage by means of the second        voltmeter.

The method may further comprise the steps of:

-   -   disconnecting the amplifier and connecting the voltage supply by        means of the switches,    -   maintaining the first bridge voltage constant with a value        different from zero, and    -   measuring the second bridge voltage by means of the voltmeter.

In addition, the method may comprise the step of measuring the fluidtemperature by means of the thermocouples.

Furthermore, the invention relates to the use of the logging tooldescribed above for determining the fluid properties of a fluid in awell downhole.

Finally, the invention relates to a detection system comprising alogging tool and a calculation unit for processing capacitancemeasurements performed by the electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its many advantages will be described in more detailbelow with reference to the accompanying schematic drawings, which forthe purpose of illustration show some non-limiting embodiments and inwhich

FIG. 1 shows a logging tool lowered into a casing of a well,

FIG. 2A shows an anemometer measuring a circuit,

FIG. 2B shows another embodiment of the anemometer measuring a circuit,

FIG. 3 shows another embodiment of the logging tool, and

FIG. 4 shows yet another embodiment of the logging tool.

All the figures are highly schematic and not necessarily to scale, andthey show only those parts which are necessary in order to elucidate theinvention, other parts being omitted or merely suggested.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a logging tool 1 in which temperatureand velocity measurements of the fluid 2 surrounding the tool downholeare conducted. In FIG. 1, the logging tool 1 is shown in a casing 3. Thetool 1 is lowered into the well and is connected with a wireline 23holding the tool in a vertical well. The logging tool 1 comprises asensor unit 5 having at least one anemometer 6 with almost the samedesign as a constant temperature anemometer. In FIG. 1, the tool 1 haseight anemometers 6 spaced apart along the circumference of the tool.

The tool 1 comprises an electrical motor which is powered through thewireline 23 to supply the sensor unit 5. The logging tool 1 may also besupplied directly through the wireline 23 without having a motor forconverting the electricity.

Each anemometer 6 has a resistance probe 7, R₁ which is connected withthe fluid 2 in the casing 3, and the heat loss in the resistance probe 7depends on the temperature of the fluid, the specific heat μ of thefluid, and the velocity v of the fluid. In order to determine one of theproperties of the fluid 2, at least three measurements must be performedat almost the same time to determine the equations and thus one of thefluid properties being the specific heat μ, velocity v and/or thetemperature T. Measurements of fluid velocity and temperature are oftenused when having long side tracks since some of these may deliver morewater than oil or other undesired elements.

The resistance probe 7, R₁ is electrically connected with three otherresistors R₂, R₃, R₄, a voltmeter V₁, and an amplifier 25 to form abridge circuit, in this case a Wheatstone bridge, as shown in FIGS. 2Aand 2B. The sensor unit 5 comprises switches 8 for disconnecting orconnecting the amplifier 25 when performing some of the measurements.The amplifier 25 is connected to two midpoints P, N on its input sideand to a midpoint M on its output side. The voltmeter V₁ is connected tothe midpoints O, M, and switches are arranged on the input side of theamplifier to enable disconnection or connection of the amplifier A. Theswitches are furthermore arranged so that they connect or disconnect asecond voltmeter V₂. In addition, a first power supply S₁, such as asignal generator, is connected to the midpoints O and M in the bridge.

In the following, the three measurements will be explained, and eventhough the measurements are referred to as a first, second and third,they may be performed in any random order.

A first measurement is performed as a normal Constant TemperatureAnemometry (CTA) where the resistance probe 7 is heated by electricalcurrent. An amplifier, such as an operation amplifier or a servoamplifier, keeps the bridge in balance so that a first bridge voltagebetween the midpoints P and N is kept at substantially zero bycontrolling the current flowing to the resistance probe 7 so that theresistance and hence the temperature are kept constant. A second bridgevoltage V₂ is measured between the midpoints O and M, and the resultrepresents how much effect is needed to keep the bridge in balance. Arepresentation of the heat transfer of the fluid is illustrated in theequations below.

The heat transferred from the probe to the fluid must be equal to theenergy conveyed to the probe by the current running through it:

$W = \overset{.}{Q}$$\frac{U^{2}}{R\left( T_{p} \right)} = {\frac{{vk}_{f}}{A}\Delta \; T}$

The resistance of the probe R=R(T_(p)) is a function of the probetemperature

, the heat transfer coefficient of the wire with surface area A is v,and the thermal conductivity of the fluid is k_(f). The potential acrossthe probe is and the temperature difference between the probe and thefluid is ΔT=T_(fluid)−T_(p).

The heat transfer coefficient is a function of velocity, meaning thatthe heat loss is velocity dependent. A commonly known consequence ofthis is the ‘wind-chill’ factor. The velocity dependency is typicallyfound to follow King's law:

$\frac{U^{2}}{R\left( T_{p} \right)} = {\Delta \; {T\left( {A + {B \cdot V^{n}}} \right)}}$

where A and B are calibration factors, V is the velocity and n<1 isanother parameter.

A second measurement is performed when the amplifier 25 is disconnectedand the second voltmeter V₂ is connected. The second bridge voltage isprovided by the signal generator S₁ as an alternating voltage, and thesecond measurement is conducted by measuring the second bridge voltageV₂ representing the alternating voltage after passing the proberesistance. A sequence of second measurements may subsequently undergo aFourier transformation to make it possible to determine the specificheat μ of the fluid 2 by comparing it with known measurements of knownfluids.

The alternating voltage may have a waveform, such as a sine, a square, arectangular, a triangle, a ramp, a spiked or a saw tooth waveform.

A third measurement is performed by disconnecting the amplifier 25 andthe second voltmeter V₂ and connecting a second power supply S₂ whilemaintaining the first bridge voltage constant at a value different fromzero by the second power supply S₂. The second measurement is performedby measuring the second bridge voltage by means of the voltmeter V₁.

The switches 8 enable the sensor unit 6 to perform three differentmeasurements at almost the same time, making it possible to determinethe fluid properties, i.e. the temperature T, the specific heat μ andthe velocity v, and it is thus unnecessary to set up any presumptions orconduct calibrations before performing a measurement in a new positionin the well.

In this way, the logging tool 1 is submerged and three measurements areperformed.

As shown in FIG. 3, the logging tool 1 may comprise a driving unit 9,such as a downhole tractor, for moving the tool forward in the well.This is particularly advantageous when performing measurements in ahorizontal part of the well.

The tool 1 has a substantially cylindrical shape with a longitudinalaxis t, and when seen in cross-section, probes 7 are arranged in theperiphery of the tool, allowing the fluid 2 to flow between the probesand the casing wall 4. The probes 7 are arranged spaced apart and withan even distance between two adjacent probes, creating a space betweenevery two probes.

In FIG. 2, the tool 1 is shown comprising electrodes 16 for measuringthe capacitance of the fluid 2. Measuring the capacitance enablesdetermination of the specific heat μ, and thus, one of the secondmeasurements may be dispensed from. Furthermore, the probes 7 are wiresextending radially outwards towards the casing wall 4.

The electrodes 16 for measuring capacitance are positioned in theperiphery of the logging tool 1. Opposite the electrodes 16, adielectric material is arranged, forming a sleeve between the well fluid2 and the electrodes. The tool 1 comprises a printing circuit (notshown). To improve the conductivity, the electrodes 16 are directlyelectrically connected to the printing circuit by means of screwsinstead of a cord.

The tool 1 may also have thermocouples 18 arranged around thecircumference 19 and the outer face 20 of the tool so that the tips ofthe electrodes 16 of the thermocouples 18 project radially from thecircumference of the tool, as shown in FIG. 4. Instead of the thirdmeasurement, the fluid temperature may be measured directly by means ofthe thermocouples.

As shown in FIG. 1, the probe 7 may also be a kind of film probe with athin-film sensor. The probe 7 may be a tungsten wire sensor, 1 mm longand 5 μm in diameter, mounted on two needle-shaped prongs. The probes 7are available with 1, 2 and 3 wires.

The voltmeter may be an analog to digital converter, also called an ADC.

By fluid or well fluid 2 is meant any kind of fluid which may be presentin oil or gas wells downhole, such as natural gas, oil, oil mud, crudeoil, water, etc. By gas is meant any kind of gas composition present ina well, completion, or open hole, and by oil is meant any kind of oilcomposition, such as crude oil, an oil-containing fluid, etc. Gas, oil,and water fluids may thus all comprise other elements or substances thangas, oil, and/or water, respectively.

By a casing 3 is meant all kinds of pipes, tubings, tubulars, liners,strings, etc. used downhole in relation to oil or natural gasproduction.

In the event that the tools are not submergible all the way into thecasing 3, a downhole tractor can be used to push the tools all the wayinto position in the well. A downhole tractor is any kind of drivingtool capable of pushing or pulling tools in a well downhole, such as aWell Tractor®.

The logging tool may also be lowered into the casing by means of coiledtubing. Although the invention has been described in the above inconnection with preferred embodiments of the invention, it will beevident for a person skilled in the art that several modifications areconceivable without departing from the invention as defined by thefollowing claims.

1. A logging tool (1) for determining the properties of a fluid (2)surrounding the tool arranged downhole in a casing (3) comprising a wall(4) and having a longitudinal extension and a longitudinal axis (t),wherein the logging tool comprises: a sensor unit (5) comprising ananemometer (6) having a resistance probe (7, R₁) electrically connectedwith three other resistors (R₂, R₃, R₄), a voltmeter (V) and anamplifier (25) for forming a bridge circuit, such as a Wheatstonebridge, having a first and a second bridge voltage, wherein the sensorunit comprises switches (8) for disconnecting or connecting theamplifier and connecting or disconnecting a voltage supply.
 2. A loggingtool according to claim 1, wherein the anemometer is a hot wireanemometer or a hot film anemometer.
 3. A logging tool according toclaim 1, wherein the probe is arranged on an outer face (20) of thetool.
 4. A logging tool according to claim 1, wherein the sensor unitcomprises a plurality of anemometers all having a resistance probe.
 5. Alogging tool according to claim 1, wherein the probes are arranged onthe outer face of the tool.
 6. A logging tool according to claim 1,wherein the tool comprises at least one thermocouple (18) arrangedpartly on the outer face of the tool.
 7. A logging tool according toclaim 1, wherein the tool comprises a plurality of electrodes (16)arranged spaced apart around the longitudinal axis in the periphery ofthe tool, enabling the fluid to flow between the electrodes and thecasing wall.
 8. A logging tool according to claim 1, further comprisinga positioning device (22) for determining a position of the logging toolalong the longitudinal extension of the casing.
 9. A logging toolaccording to claim 1, wherein a temperature of the probe resistance ismaintained constant by varying the current by means of the amplifier sothat a first bride voltage is substantially zero when a firstmeasurement of a second bridge voltage is performed by the voltmeter,and the second bridge voltage is an alternating voltage when a secondmeasurement of the first bridge voltage is performed.
 10. A logging toolaccording to claim 9, wherein the second bridge voltage is maintainedconstant at a value different from zero by the voltage supply when athird measurement of the first bridge voltage is performed.
 11. Alogging tool according to claim 9, wherein a third measurement isperformed using the thermocouples.
 12. A logging tool according to claim9, wherein the alternating voltage or alternating current have sine,square, rectangular, triangle, ramp, spiked or sawtooth waveforms.
 13. Amethod for determining the properties of a fluid (2) by means of thelogging tool according to claim 1, comprising the steps of: submergingthe logging tool into a casing, maintaining a constant temperature inthe resistance probe by varying the current, increasing the bridgecurrent by means of the amplifier if the temperature in the resistanceprobe decreases until a first bridge voltage is substantially zero,measuring a second bridge voltage by means of the voltmeter,disconnecting the amplifier and the voltage supply and connecting asecond voltmeter by means of the switches, providing the second bridgevoltage as an alternating voltage, and measuring the first bridgevoltage by means of the second voltmeter.
 14. Use of the logging toolaccording to claim 1 for determining the fluid properties of a fluid ina well downhole.
 15. A detection system comprising a logging toolaccording to claim 1 and a calculation unit for processing capacitancemeasurements performed by the electrodes and/or a driving unit formoving the tool forward in the casing.